1. Field of the Invention
This invention relates generally to a power system for a hybrid fuel cell vehicle and, more particularly, to a power system for a hybrid fuel cell vehicle that employs a floating base load strategy that reduces fast power transient demands from the fuel cell power module.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
Most fuel cell vehicles are hybrid vehicles that employ a rechargeable electrical energy storage system (EESS) in combination with the fuel cell stack, such as a DC battery or a super-capacitor. The EESS provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to an electric traction system (ETS) and other vehicle systems through a DC voltage bus line for vehicle operation. The EESS provides supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. Therefore, the EESS would provide the extra 30 kW of power. The fuel cell stack is used to recharge the EESS during those times when the fuel cell stack is able to meet the system power demand. The generator power available from the ETS during regenerative braking is also used to recharge the EESS through the DC bus line.
As discussed above, the power demand from the ETS can be provided by the fuel cell stack, the EESS, or a combination of both. Normally, the EESS can provide energy faster than the fuel cell stack, and therefore can also increase the dynamic capabilities of the vehicle. Also, the fuel cell system can be made smaller and still provide the same driving capabilities, or the dynamic requirements of the fuel cell system can be reduced, which can increase durability.
For a typical hybrid vehicle strategy, the EESS is mainly used to increase efficiency, to lower the dynamic requirements of the fuel cell system, and/or to increase the performance of the vehicle. If the ETS demands more power, the EESS can provide the stored energy to the ETS very fast. Additional demanded power can be quickly provided by the fuel cell system.
The fuel cell system power demand for certain vehicle drive cycles may require that the fuel cell system operate in very different and fast changing power levels with high power gradients. These frequent changes in power may cause many voltage changes in the stack output power that reduces the lifetime and durability of the stack. In addition, fuel cell system components are highly stressed during hard power transients of the fuel cell stack. Therefore, a reduction of fast voltage changes will improve the durability of the fuel cell stack.